Foaming Performance of Linear Polypropylene Ionomers

Abstract

Linear polypropylene (PP) exhibits poor foamability because of its inherently low melt strength. Incorporation of long-chain branches is a common approach to increase melt strength; however, it is a costly process. To bridge this gap, we set out to study the effect of ionic content on the foaming behavior of linear PP ionomers using a PP homopolymer as a control and a commercial high melt strength long-chain branched PP as a benchmark. Small-angle X-ray scattering confirms the presence of ion clusters in the ionomers, which act as physical cross-linking points as was recently shown, providing enhancement to the dynamic moduli, complex viscosity, and strain hardening behavior, even at ion contents below 0.1 mol %. We found that, upon foaming, the PP ionomer with a higher ionic content exhibited higher foam uniformity, higher cell density (∼108 cells/cm3), and smaller cell size (as small as 37 μm) compared to those of its linear counterpart. Furthermore, the linear ionomer resin and the commercial branched PP showed similar foam morphologies and mechanical responses under cyclic indentations over the temperature range 155–175 °C. These findings reveal a new, and potentially cost-effective, route to produce high performance foams using linear PPs that are compliant to industrial standards.